Axisymmetric poloidal magnetic field described by cylindrical coordinates and local orthogonal curvilinear coordinates. The angle ϕ representing the angle of rotation around the vertical symmetry axis, z, is not shown. The figure shows distance, s, along a curved magnetic field; the angle, θ, between the poloidal field and the vertical axis; and incremental increases δs and δa, along and across the poloidal field, respectively.